1. Field of the Invention
The present invention relates to a method for driving a liquid crystal display device which comprises a pixel region constituted of a display pixel region in which a plurality of pixels are arranged in matrix and a dummy pixel region arranged in the periphery of the display pixel region.
2. Description of the Related Art
FIG. 1 is a plan view for showing a pixel region in a conventional liquid crystal display device driving method. Description will be provided hereinafter by referring to the drawing.
In order to make an optical property of the entire display pixel region 1, in which a number of pixels are arranged in matrix, uniform for a liquid crystal applied voltage of the entire display pixel region 1 in which a number of pixels are arranged in matrix, a dummy pixel region 2 which does not directly contribute to a picture display is provided in the outer periphery of the display pixel region 1. Further, in the driving method, the voltage to be applied to a pixel electrode of the dummy pixel region 2 is set to be the maximum value of a video signal voltage which is applied to the pixel electrode of the display pixel region 1. The reason will be described in the followings.
FIG. 2 is a cross section taken along the line X of FIG. 1. Description will be provided hereinafter by referring to the drawing.
Lines illustrated within a liquid crystal layer 33 show electric flux lines which are generated when the same voltage as that of a counter electrode 34 is applied to a dummy pixel electrode 31 and the maximum value of the video signal voltage is applied to a display pixel electrode 32 of the display pixel region 1. In the state where the voltages are applied in the manner as described above, a transverse electric field is generated in the liquid crystal layer 33 in a boundary area 35 between the dummy pixel region 2 and the display pixel region 1. Thus, liquid crystal molecules are in a laid position (that is, facing the sideways). Therefore, the transmissivity of the liquid crystal layer 33 in the vicinity of the boundary region 35 becomes different from that of the center area of the display pixel region 1, thereby deteriorating the display quality. More specifically, in the case of a normally white system which displays white when a voltage is not applied to the liquid crystal of the liquid crystal layer 33, if a voltage is applied to display black over the entire display pixel region 1 and to display white in the dummy pixel region 2, the periphery of the display pixel region 1 looks whitish due to a leakage of the light.
In order to avoid the above-described phenomenon, the maximum value of the voltage to be applied to the display pixel electrode 32 may be applied to the dummy pixel electrode 31. This can be supported by Japanese Patent No. 2590992 (FIG. 5, 47-50 lines in right section on page 2).
However, as in the related art as described above, when the voltage to be applied to the dummy pixel electrode 31 is set to be the maximum value of the video signal voltage which is applied to the display pixel electrode 32, a reverse twisted domain is generated within the dummy pixel region 2. And if the influence spreads to the display pixel region 1, it causes a defective indication. The defective indication will be described in the followings by referring to a case of using a gate line inversion driving method.
The reverse twisted domain is generated from the state where the liquid crystal molecules are in a rise-up state, and it is more likely to be generated when the extent of the rise of the liquid crystal molecules is prominent. In other words, it is more likely to be generated when the higher voltage is applied to the liquid crystal layer 33.
The dummy pixels within the dummy pixel region 2 do not have apertures, that is, the entire dummy pixels are covered by a shield film so that there is almost no photoelectric current leakage generated from a switching element (referred to as TFT (thin film transistor) hereinafter) contained in the dummy pixel.
Therefore, even when the same voltage as that of the display pixel region 1 is applied to the dummy pixel region 2, the higher voltage is maintained in the dummy pixel region 2 after one frame period, compared to the display pixel region 1 which has the apertures. Thus, in the dummy pixel region 2, the liquid crystal molecules rise up to a larger extent. Moreover, the maximum voltage to be applied to the display pixel electrode 32 is continued to be applied to the dummy pixel region 2 constantly so that the liquid crystal molecules always maintain the rise-up state.
Since the polarities of the voltage to be applied to the liquid crystal are changed for each line of the pixel matrix in the gate line inversion driving method, there are transverse electric fields generated between the pixel electrodes in the vertical direction of the screen provided that a plurality of gate lines are arranged on the screen in parallel in the vertical direction. The liquid crystal molecules in the region of the transverse electric field are likely to cause abnormal orientation, so that it is likely to generate the reverse twist. When there is the reverse twisted domain generated between the pixels on the neighboring gate lines within the dummy pixel region 2, the influence of the reverse twisted domain spreads to the peripheral liquid crystal molecules. The reverse twisted domain propagates to the display pixel region 1 from the dummy pixel region 2. That is, in the case of the gate line inversion driving method, the reverse twisted domain generated within the dummy pixel region 2 propagates to the display pixel region 1 along the gate line, thereby causing the defective indication with horizontal lines being generated in the display pixel region 1 along the gate line.